Biodegradable Plastic Agricultural Mulches: Microbial Degradation and Implications for Soil Health.

Agricultural plastic mulch films have multiple agronomic benefits. Conventional plastic mulch films made of polyethylene are not readily biodegradable, and thus impose considerable end-of-life costs in terms of removal and disposal, as well as environmental impacts associated with improper disposal or fragmentation. Biodegradable plastic mulches (BDMs) present a viable alternative to PE that can be tilled into the soil after the growing season. However, decomposition and fate of BDMs and their impact on soils is not well understood; this is a primary concern with growers and regulators. The goal of our study was to 1) determine impacts of BDMs on soils, emphasizing microbial communities and carbon cycling; and 2) understand microbial degradation of BDMs. In randomized block trials with four BDMs and conventional polyethylene film, BDMs altered soil microclimate comparably to polyethylene mulches. After two seasons, there was no significant impact on overall soil quality as a result of tilling mulches into soil and only minor changes observed in the soil microbial communities. To better understand the degradation of these materials in soil, lab microcosms and enrichment cultures were used. In soil microcosms, degradation of BDMs was enhanced under higher temperature with the percent biodegraded more than doubling with each ten degree rise in temperature. Microscopy confirmed increased microbial growth on films at higher temperatures. Enrichments on plastics selected for a degradative community, and we present the first characterization of a BDM “plastic-ome”: a metagenomic profiling of the microbial communities associated with biodegradable mulch films. Strains isolated from enrichments included Rhodococcus, Bacillus, and Streptomyces spp. which were capable of degrading multiple BDMs in culture, but appear to have different metabolic strategies for degradation. Taken together, this work identified potential degraders and demonstrated microbially-mediated degradation of BDMs, providing a foundation for understanding mechanisms of biodegradable plastic catabolism by soil microbes.